This invention relates to inductors, and more particularly, to inductors formed on integrated circuits.
Integrated circuits are used in a variety of electronic equipment. It is often necessary to use inductors with such circuits. For example, the circuitry used for radio-frequency (RF) applications, microwave applications, and many analog circuit applications can require inductors.
Although it is sometimes possible to make use of discrete inductors, inductors that are formed on the integrated circuit itself are often preferred or required for performance and economic reasons. Fabricating an inductor as part of an integrated circuit structure eliminates the need for special packaging and can enhance reliability. Integrated circuit inductors are typically spiral in shape and are formed on the top surface of the circuit.
An important figure of merit for an integrated circuit inductor is its quality factor Q. The quality factor Q for an inductor is the ratio of the energy stored in the inductor to the energy dissipated by the inductor. Inductors with low Q values are inefficient and can degrade circuit performance.
The value of Q is affected by the resistance of the conductive lines used to form the inductor. Conductors with relatively higher resistance produce inductors with poor Q values. As a result, various approaches have been made to reduce the resistance of the lines.
With one approach, an upper-layer aluminum line is connected to a lower-layer aluminum line by tungsten vias. The lower-layer aluminum line serves as a parallel current path that can help to reduce the resistance of the upper-layer aluminum line. However, tungsten is not highly conductive and the connections between the upper-layer aluminum line and lower-layer aluminum line are only made at a finite number of locations, so the resistance of the inductor lines formed in this way can still be higher than desired.
Another approach for reducing the resistance of the lines involves adding additional thickness to the lines. However, many integrated circuit processes do not allow thick metal lines to be formed. Although lines can sometimes be thickened using special post-processing procedures, the resulting increase in process complexity can have an adverse impact on cost and reliability.
It might be desirable to reduce the resistance of the conductive lines by increasing the width of the conductive lines. However, with a normal semiconductor fabrication process there is generally a maximum line width allowed by the process design rules. Moreover, regardless of the process that is used, the line width cannot be too large, because this can lead to capacitive coupling with the silicon substrate, which reduces Q.
It is therefore an object of the present invention to provide integrated circuit inductors with improved Q values.